The present invention relates to a power converter such as an inverter or a power storage, and particularly relates to a power converter of a pulse width modulation control system with an electric current detector using a shunt resistor.
Inverters have been used widely for operating AC motors such as induction motors, and recently used also as controllers for power sources of carriages. Thus, the advantage of adjustable speed operation by using the inverters may be enjoyed sufficiently.
For the control of an inverter, detection of a load current may be required. A hall element type current sensor 28 or a shunt resistor 13 with a detection circuit 18 has been conventionally adopted for the detection of the load current.
The hall element type current sensor 28 is a current sensor in which a hall element is provided in a part of an annular magnetic substance, and an electric wire supplied with a load current is wound around or passed through the magnetic substance so that magnetic flux generated by the load current is converted into a voltage by the hall element. In this case, there is a merit that a detection signal electrically isolated from an electric circuit which is a target to be detected can be obtained.
Similarly, the shunt resistor 13 and the detection circuit 18 are defined as follows. That is, a resistor inserted in series into an electric circuit in which a load current flows is a shunt resistor. A voltage drop appearing between the opposite terminals of the shunt resistor 13 due to the load current. In order to form a detection signal from the voltage drop, a circuit is used as the detection circuit 18. The shunt resistor 13 with the detection circuit 18 can be provided with considerably low cost. Thus, such circuits have been conventionally used widely.
FIG. 11 shows an example of the background art in which both the hall element type current sensor 28 and the shunt resistor 13 with the detection circuit 18 have been applied to a power converter of a PWM (Pulse Width Modulation) control system as a target. Here, both the hall element type current sensor 28 and the shunt resistor 13 with the detection circuit 18 are illustrated for the sake of description. Actually, it will go well if either the hall element type current sensor 28 or the shunt resistor 13 with the detection circuit 18 is provided.
In FIG. 11, a main circuit is constituted by a converter (power rectification portion) 14, an inverter (power inversion portion) 15 of a PWM control system, and a smoothing capacitor 16. The converter 14 is constituted by a diode rectifier. DC power outputted from the converter 14 is supplied to the inverter 15. The capacitor 16 is connected to a DC circuit between the converter 14 and the inverter 15.
Then, when AC power is supplied to the converter 14 from a commercial power source 29 as a power source, DC power smoothed by the capacitor 16 is supplied to the inverter 15. Here, semiconductor switching devices 5 represented by IGBTs (Insulated Gate Bipolar Transistors) in the inverter 15 are PWM-controlled so that the DC power is converted into AC power with a specific voltage and a specific frequency. As a result, power with a variable voltage and a variable frequency is supplied to a load such as an induction motor.
As shown in FIG. 12, also in a power converter in which DC power outputted by a power storage 30 such as a battery is supplied to the inverter 15, semiconductor switching devices 5 of the inverter 15 are PWM-controlled in the aforementioned manner so that DC power is converted into AC power with a specific voltage and a specific frequency. As a result, power with a variable voltage and a variable frequency is supplied to a motor 17 as a load, such as a power source for a carriage, a cooling fan of a cooler, a pump driving motor for circulating cooling water, a hydraulic pump driving motor for hydraulic apparatus, or a compressor driving motor for an air conditioner.
At this time, on (conduction) and off (interruption) of the semiconductor switching devices 5 in the inverter 15 are controlled in accordance with PWM signals by a computer 19 through a driver circuit as shown in FIG. 11. To this end, the value of a current flowing in the motor 17 which is a load, that is, the value of a load current is required for the control by the computer 19.
To detect the value of the load current, there are two methods as described previously. That is, one is a method using the hall element type current sensor 28 and the other is a method using the shunt resistor 13 and the detection circuit 18.
First, when the hall element type current sensor 28 is used, this current sensor is connected in series between the inverter 15 and the motor 17 which is a load. The detection result by the current sensor 28 is A/D converted and supplied to the computer 19.
On the other hand, when the shunt resistor 13 and the detection circuit 18 are used, the shunt resistor 13 is connected in series between the capacitor 16 and the inverter 15. A voltage drop appearing due to a load current flowing in the shunt resistor 13 is A/D converted through a filter, an amplifier and so on, and supplied to the computer 19. The shunt resistor 13 may be connected in series between the inverter 15 and the motor 17.
The shunt resistor 13 is, generally, of a sheet-like resistive material 6 made of manganin material (alloy of copper and manganese) excellent in temperature characteristics. The sheet-like resistive material 6 is formed into a predetermined shape by punching or by etching after fixed attachment to an insulating layer 4. A shunt resistance 8, main electrodes 7 for making a load current flow into the shunt resistance 8, and detection electrodes 31 for detecting a voltage generated in the shunt resistance 8 are formed from the same resistive material. As shown in FIGS. 13A and 13B, the shunt resistance 8, the main electrodes 7 and the detection electrodes 31 are mounted on a heat radiating base plate 1 of a power module through the insulating layer 4. The heat radiating base plate 1 is superior in heat radiation characteristics, and the semiconductor switching device 5 of the inverter 15 is mounted on the base plate 1.
Heat generation in the shunt resistor 13 occurs in both the shunt resistance 8 and the main electrodes 7 because the load current flows into the shunt resistance 8 and the main electrodes 7. The generated heat flows into the heat radiating base plate 1 so that the temperature increase is suppressed.
The length, width and thickness of the shunt resistor 13 using the sheet-like resistive material 6 are defined as follows. That is, the length direction of the shunt resistor 13 is defined as the direction in which a detection current flows. The width direction of the shunt resistor 13 is defined as the direction perpendicular to the length direction. The thickness direction of the shunt resistor 13 is defined as the direction perpendicular to the insulating layer 4.
In the background art, the hall element type current sensor 28 or the shunt resistor 13 with the detection circuit 18 is used for PWM control of a power converter. However, in the case of the hall element type current sensor 28, a comparatively expensive hall element and a large magnetic substance are required. Accordingly, there is a problem in cost reduction and miniaturization.
On the other hand, the shunt resistor 13 and the detection circuit 18 can be constituted by small and inexpensive electronic parts. However, the shunt resistor 13 and the detection circuit 18 are connected in series with a power line so as to detect a load current ranging from several amperes to several thousands of amperes. Thus, heat is generated. Although manganin material or the like having a low rate of resistance temperature change is used to improve the accuracy, the resistivity of the manganin material is several tens of times as high as that of copper material. Thus, the resistance value is required to be made a minimum (about 0.5-0.6 mxcexa9) in order to suppress heat generation. When the sheet-like resistive material 6 is made thick and short, the bottom area of the resistive material is reduced. As a result, the heat radiation resistance increases to cause temperature increase. Further, heat is generated also in the shunt resistance 8 and the main electrodes 7 for making the load current flow into the shunt resistance 8. Thus, the amount of heat generated in the shunt resistor as a whole is increased. Indeed such increase of the amount of heat generated in the shunt resistor as a whole can be avoided by increasing the size of the main electrodes 7 to thereby increase the heat radiation area and reduce the thermal resistance. But the shunt resistor 13 and the detection circuit 18 cannot be made small in size and low in cost. Thus, it is difficult to apply the shunt resistor 13 and the detection circuit 18 to a high-capacity power converter. Further, the load current does not flow into the shunt resistor 13 uniformly by electromagnetic induction caused by the layout of power wiring to the shunt resistor 13. Thus, there arises a problem that heat generation is concentrated or the detection accuracy deteriorates.
An object of the invention is to provide a power converter excellent in control characteristics, small in size and low in cost, by use of an accurate and compact shunt resistor whose thermal resistance is reduced so that the system for detecting a load current by the shunt resistor and a detection circuit can be applied to a high-capacity power converter, while the load current distribution in the shunt resistor is made uniform.
The foregoing object is attained by a power converter in which a load current is detected from the shunt resistor 13 constituted by three parts, that is, the shunt resistance 8 and two main electrodes 7. In the power converter, the shunt resistance 8 and the main electrodes 7 are formed out of one and the same sheet-like resistive plate 6. One side of the shunt resistor 13 is fixedly attached to the insulating layer 4. Sheet-like plates 9 are fixedly attached to side surfaces of the main electrodes 7 opposite to the side surfaces with which the main electrodes 7 are fixedly attached to the insulating layer 4. Each of the plates 9 is lower in volume electric resistivity than at least the sheet-like resistive plate 6 and thicker in thickness than the sheet-like resistive plate 6. Plate main electrodes 11 are provided on the plates 9, while plate detection electrodes 12 are provided on the plates 9 for detecting a voltage generated in the shunt resistance 8 and electrically connecting the detected voltage to another wiring. Accordingly, heat generated in the two main electrodes 7 formed out of the same sheet-like resistive plate 6 as the shunt resistance 8 can be reduced on a large scale. Further, since the detection electrodes are provided on the plates 9 having low volume electric resistivity, the power converter can be made smaller in size and higher in accuracy.
The foregoing object is attained by a power converter in which a load current is detected from the shunt resistor 13 constituted by three parts, that is, the shunt resistance 8 and two main electrodes 7. In the power converter, the shunt resistance 8 and the main electrodes 7 are formed out of one and the same sheet-like resistive plate 6. One side of the shunt resistor 13 is fixedly attached to the insulating layer 4. Sheet-like plates 9 are fixedly attached to the other surfaces of the main electrodes 7 respectively. Each of the plates 9 is lower in volume electric resistivity and higher in thermal conductivity than at least the sheet-like resistive plate 6 and thicker in thickness than the sheet-like resistive plate 6. Plate main electrodes 11 are provided on the plates 9, while plate detection electrodes 12 are provided on the plates 9 for detecting a voltage generated in the shunt resistance 8 and electrically connecting the detected voltage to another wiring. Accordingly, heat generated in the shunt resistance 8 can be thermally transmitted and thermally diffused by use of the plates 9. Thus, the thermal resistance of the shunt resistor 13 can be reduced.
The foregoing object is attained by a power converter in which a load current is detected from the shunt resistor 13 constituted by three parts, that is, the shunt resistance 8 and two main electrodes 7. In the power converter, the shunt resistance 8 and the main electrodes 7 are formed out of one and the same sheet-like resistive plate 6. One side of the shunt resistor 13 is fixedly attached to the insulating layer 4. Sheet-like plates 9 are fixedly attached to the other surfaces of the main electrodes 7 respectively. Each of the plates 9 is lower in volume electric resistivity and higher in thermal conductivity than at least the sheet-like resistive plate 6 and thicker in thickness than the sheet-like resistive plate 6. Plate main electrodes 11 are provided on the plates 9, while at least one constricted portion 23 for narrowing a current flow path is provided between each of said main electrodes 11 for the plate main electrodes 11 and the shunt resistance 8 so as to be located in the plates 9 and the main electrodes 7. Accordingly, the flow path of a load current in the respective plates 9 and the respective main electrodes 7 can be controlled. Thus, the current density of the load current flowing into the shunt resistance 8 can be made uniform.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.